The mechanism of release of active alpha subunit from dimeric alpha beta AMV DNA polymerase has been elucidated. The alpha subunit was released upon direct but limited exposure of the purified alpha beta enzyme to CM-bound trypsin matrix. Electrophoretic analysis of digested enzyme revealed a progressive fragmentation, with simultaneous increase in the alpha subunit and decrease in beta subunit. Stochiometric analysis indicates that there are two active groups essential for activity with the alpha subunit and one with the alpha beta. We have shown the presence of type-C virus in lymphosarcoma in fish (Northern pike, Esox lucius). The reverse transcriptase associated with the virus has many similar characteristics known to many transcriptases of mammalian or avian origin. The striking difference between the pike enzyme and other polymerases is its low temperature optimum activity profile which may play a role in the seasonal nature of the appearance of the disease.